Insulating tubular conduit apparatus

ABSTRACT

A concentric insulating tubular conduit member for use in forming a conduit string, such as a tubing string in a subterranean well, is disclosed. Each individual concentric conduit comprises an outer tubing and an outwardly flared inner tubing welded to the outer tubing at the end of the flared section. The inner tubing member is formed from an initially straight cylindrical member having enlarged ends and, when outwardly flared, the flared ends, although stretched, have a thickness at least equal to the nominal thickness of the inner tubular member. Insulation may be contained within the annular cavity between the inner and outer tubing members and between inner and outer coupling members at the juncture between adjacent tubing sections.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation application of my co-pendingapplication Ser. No. 272,411, filed June 10, 1981, now U.S. Pat. No.4,396,211, entitled "Insulating Tubular Conduit Apparatus and Method".

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention generally relates to an insulated conduit havingparticular utility in subterranean wells and more particularly to aconcentric walled insulated conduit having an annular space between thewalls within which an insulating material is deposited and sealedtherein.

2. Description of the Prior Art

In producing some subterranean wells, steam is injected into aninjection well to increase recovery of hydrocarbons by reducing highviscosity crude oil, otherwise known as "heavy crude". The lowerviscosity makes the oil more readily pumpable. One technique for doingthis is to inject a high quantity of steam into the production zonecontaining "heavy crude" for an extended period of time, such as fromabout three to about five weeks. At that point, the viscosity of theheated crude will be reduced and will be readily pumpable through aproduction well in communication with the production zone. The injectionwell may also be modified for production. A steam "flood" may also beprovided by known techniques, generally through an injection well, todrive the flood and the produced hydrocarbons into a nearby productionwell.

One of the major problems in injecting steam into a subterraneanproduction zone through conventional well production tubing is that thesteam loses a large quantity of its heat to the well bore casing andsurrounding formation as it travels downwardly to the production zone.Attempts have been made in the past to reduce the heat loss of steamintroduced into subterranean formations. One such attempt is disclosedin U.S. Pat. No. 3,511,282, issued on May 12, 1970. This patentdiscloses a dual-wall tube structure having insulation sealed in theannulus between the inner and outer walls by bushings respectivelywelded at each end between the inner wall and the outer wall. The innerwall is prestressed in tension prior to being welded to the outer wall.The space defined between the inner and outer walls is filled with aconventional insulating material such as calcium silicate. Although thistechnique may be satisfactory in some oil field installations, it is notsatisfactory for all oil field installations where large temperaturedifferentials are encountered between the inner and outer walls. In thiscase, even though the inner wall is prestressed in tension, the innerwall, as it is heated, will elongate with respect to the outer wall sothat the inner wall may even change from a tension to a compressioncondition with the attendant danger of buckling. The magnitudes of theforces generated are such that localized stresses are created in theweld areas causing cracks which permit exposure of the insulation towell fluids and eventually causing failure or degradation of theinsulating structure. Centralizers are incorporated to reduce buckling,but may also, in turn, contribute to a loss of heat because of thegenerally durable nature of such devices.

Another known technique of handling the aforedescribed inner and outerwalls of an insulating tube is to place a thin walled bellows betweenthe two walls at each end of the assembly, one end of each of thebellows being rigidly attached to the inner wall, and the other end ofthe bellows being rigidly attached to the outer wall. This technique, ofcourse, relieves the strain on the welds and joining structure betweenthe walls due to the relative movement between the inner and outerwalls. However, the bellows introduce other problems; namely, thebellows are comparatively thin walled and delicate, being typicallyformed from a heat resistant, springy material, which cannot withstandthe rough handling normally encountered in the oil patch.

A concentric walled thermal insulating conduit is also disclosed andclaimed in U.S. patent application, Ser. No. 264,728, entitledCONCENTRIC WALLED CONDUIT FOR TUBULAR CONDUIT STRING. This tubularconduit comprises concentric tubular members in which the inner tubularmember is corrugated and has flared ends welded to the outer tubularmember adjacent each end. Insulation is provided within the annular areabetween the two tubular members for reducing heat loss during steaminjection.

SUMMARY OF THE INVENTION

A concentric walled insulating tubular conduit for forming a tubularstring in a subterranean well has an inner tubing member with flaredends. The flared ends of the tubing, with a thickness at least equal tothe nominal thickness of the inner tubing intermediate the ends, iswelded to the outer tubing. Only two welds per individual conduit arenecessary. The flared inner ends are fabricated by forging the ends of atubular member having upset or enlarged ends. Although the thickness ofthe ends will be reduced by the forging operation, the flared ends willremain relatively thick, thus adding to the integrity of the weld. Theinner tubing member is preferably prestressed in tension relative to theouter tubular member.

An exterior coupling joins adjacent members by conventional threadedengagement with the outer tubing and an inner coupling member extendsbetween flared sections of the inner tubing ends. Insulation may beincorporated between the interior and exterior coupling and in theannular cavity between the inner and outer tubing. Blanket insulation,rigid load-bearing insulating members, and a shield having a low thermalemmissivity are provided in the axially extending cavity.

The concentric walled conduit is preferably fabricated by using standardtubular members used in oil and gas wells, the inner tubing originallyhaving upset ends. Fabrication utilizing these standard tubular membersyields a concentric walled tubular member in which only two welds wouldbe necessary on each conduit section for joining the flared end sectionsof at least nominal tubing thickness to the outer tubing. These flaredend sections, however, remain relatively long and thin, thus reducingthe path available for conductive heat transfer.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic illustrating the injection of steam through atubing string formed of individual conduit members constructed inaccordance with this invention.

FIG. 2 shows two conduits coupled at their ends and this section viewdepicts the components of the preferred embodiment of this invention.

FIG. 3 illustrates the forging operation in which a conventional upsettubular is flared by use of a swage to form the inner tubing of theconcentric insulating member.

FIG. 4 shows the profile of an upset tubular after the ends have beenflared for use as the inner conduit of the concentric walled tubularassembly.

FIG. 5 depicts the means of forming a vacuum within the annularinsulating cavity in this invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT

FIG. 1 illustrates, in schematic form, the use of a plurality ofsections comprising concentric walled insulating tubing members formedin accordance with this invention, to construct an insulating tubingstring. The tubing string T, shown in FIG. 1, permits the injection ofsteam at the surface of the well through the tubing to the formationtherebelow. The insulating tubing string ensures that the heat lossbetween the surface and the formation will not be so excessive as todefeat the function of steam injection. The tubing string T, comprisinga plurality of individual insulating tubing conduits 2, is positionedwithin the well and within the well casing C in much the same manner asa conventional tubing string.

FIG. 2 shows the components of each individual conduit and theinterconnection between adjacent abutting tubular conduits. It will beunderstood that the opposite ends of each individual conduit is ofgenerally the same configuration as is shown in FIG. 2. Each individualconcentric insulating member 2 comprises an outer tubing 4 and an innertubing 6. The outer tubing 2 comprises a straight cylindrical memberhaving conventional threads 10 at each end. A conventional externalcoupling 8, engaging threads 10, can be used to join adjacent concentricmembers. In order to reduce the number of welds needed to secure innertubing 6 to outer tubing 4, the end of inner tubing 6 is outwardlyflared, as shown in FIG. 2. A single circular face weld 30 can then bemade between inner tubular 6 and outer tubular 4. The reforming of innertubing 6 results in flared ends having substantially three sections.First outer section 32 generally comprises a radiused portion having aneffective radius of curvature approximately equal to or on the order ofthe separation between the inner and outer tubing. The radius ofcurvature need not be limited to this separation distance, but adesirable structure can be constructed by employing a radius ofcurvature of that order of magnitude. As shown in FIG. 2, the thicknessof this radiused portion would be generally equal to a value, D₃.Adjacent the outer radiused section 32 on each end of inner tubing 6, isa tapered section 34. The degree of taper in this section need not belarge and, in the preferred embodiment of this invention, a radiallyoutward taper of 1° is employed in section 34. In the preferredembodiment of this invention, a second more significantly taperedsection 36 is employed to form a transition between the 1° taperedsection 34 and the central portion of the inner tubing 6. Thistransition section 36, in the preferred embodiment of this invention,has a taper equal to approximately 5°.

In the assembled configuration of a single insulating tubing conduit, asshown in FIG. 2, an annular cavity 13 is formed between outer tubing 4and inner tubing 6. This annular cavity 13 may be filled with thermalinsulation. In the preferred embodiment of this invention, this thermalinsulation comprises a combination of a blanket insulation 12 havingceramic fibers, at least one rigid insulating member 14, and areflective heat shield 18. At least one rigid cylindrical insulatingmember 14 is located within annular cavity 13 between the welded endsjoining outer tubing 4 to inner tubing 6. In the preferred embodiment ofthis invention, this rigid insulating member comprises a molded, hightemperature pipe and block insulating composed by hydrous calciumsilicate. This molded calcium silicate member 14 provides structuralsupport between inner tubing member 6 and outer tubing member 4 betweenthe ends of annular cavity 13. In the preferred embodiment of thisinvention, insulating member 14 comprises a conventional pipe and blockinsulating member which is commercially available. One molded calciumsilicate pipe and block insulation member that can be used in thisinvention is manufactured by Johns-Manville and is commonly refered tounder the trademark "Thermo-12". These standard pipe and blockinsulation members are available in half-sections which can bepositioned in surrounding relationship with respect to inner tubing 6.Metal bands 16 can then be attached annular insulating memberstructurally supporting the outer tubing 4 relative to the inner tubing6.

The remainder of annular cavity 13 contains a blanket insulation 12,which is also commercially available. Thermal insulating blankets,composed of long mechanically bonded refractory fibers providing acombination of high blanket strength, flexibility and high thermalperformance, are commercially available. In the preferred embodiment ofthis invention, a thermal insulating blanket of the type manufactured byJohns-Manville under the trademarks "Thermo-Mat" or "Ceratex", has beenemployed to form a convective insulating barrier within annular cavity13. This insulating blanket can be secured to the inner tubing betweencalcium silicate insulating members 14 and the ends of annular cavity13. This insulating blanket 12 can be secured to inner tubing 4 bywrapping a conventional glass fiber tape around the exterior of theinsulating blanket 14. When employed in combination, blanket insulation12 and the rigid calcium silicate insulating member 14 shouldsubstantially fill annular cavity 13 between the inner and outertubings. In the preferred embodiment of this invention, at least apartial vacuum is established in annular cavity 13 to prevent moisturefrom degrading the performance of the convective insulation.

In addition to the convective insulating barriers provided by blanketinsulation 12 and rigid insulating member 14, a radiant reflective heatshield member 18 can be provided. In the preferred embodiment of thisinvention, this reflective heat shield is incorporated on the outersurface of inner tubing 6, and comprises a material having a relativelylow thermal emissivity. In this embodiment, aluminum foil has beenapplied around inner tubing 6. This aluminum foil comprises a reflectivesurface which will further reduce the heat transfer of this tubingassembly.

Annular cavity 13 provides sufficient space to contain insulation formaintaining appropriate heat transfer characteristics over most of thelength of this tubing. There does, however, remain a space betweeninterior flared ends on adjacent tubing members. An interior coupling orcylindrical spacer member 20 can be employed to completely isolate thearea otherwise bounded by the flared inner tubing ends of adjacentconduits and the outer coupling 8. This interior coupling 20 comprises acylindrical member having outer sections 24 and 26 having a thicknesswhich is less than the thickness of the central section 28 of theinterior coupling member. As shown in FIG. 2, the ends 24 and 26 can bewedged into engagement with the tapered section 34 of each inner tubingmember 6. Insulation can then be positioned around the exterior ofinterior coupling 20 to reduce heat loss in the vicinity of thecoupling. In the preferred embodiment of this invention, blanketinsulation of the same type as blanket insulation 12 used within annularcavity 13 can be affixed around interior coupling central section 28 ina donut fashion. The blanket insulation then fills the cavity bounded bythe radiused ends of adjacent interior tubing members and the interiorand exterior coupling members. An assembled tubing string or conduitcomprising a plurality of individual insulating tubing conduits 2 wouldthen have insulating material positioned within the annular spacebetween inner tubing 6 and outer tubing 4 along substantially the entirelength of the insulating tubing conduit 2. Finally, a second lowemissivity barrier or radiant heat shield is provided on the exterior ofthe outer tubing. The outer tubing can be painted along its entirelength to provide this barrier. Two low emissivity barriers will thenact to reduce heat transfer over most of the tubing.

The flared ends of inner tubing 6 not only provide an effective means ofincreasing the performance of the welds, both by reducing their numberand by increasing the welded area, but they should also provide for lowheat loss by means of conduction through the welded joint. The only heatconductivity path between the junction of the interior coupling 20 andthe tapered portion 34 of inner tubing 6 is along the relatively longthin flared tubing itself. No relatively wide bushing member with itsinherently greater heat conductivity is necessary. Still, the flaredportion of the tubing is thick enough to provide a weld of highintegrity.

FABRICATION

One very significant feature of the preferred embodiment of thisinvention is that it can be fabricated using only conventional andcommercially available components. While the concentric conduit 2 can befabricated by using a wide variety of cylindrical members, the preferredembodiment of this invention can be fabricated by using standardAmerican Petroleum Institute tubulars. In one size, this invention mayutilize a standard 23/8ths inch O.D. A.P.I. J-55 tubing having upset orenlarged ends to allow fabrication of the flared inner tubing 6. In thesame configuration, a 41/2 inch A.P.I. J-55 casing having non-upset endscan be employed for outer tubing 4. The standard tubing shown in FIG. 3,such as 23/8ths inch O.D. J-55 tubing, has a nominal thickness D₁ alongmost of the tubing. This nominal thickness D₁ is less than the thicknessD₂ of the upset ends. The ends of the standard J-55 tubing can be flaredto their final configuration by utilizing a forging operation employinga swage 42, shown in FIG. 3. The swage has a beveled portion 44 at itsend. Adjacent this beveled entry surface 44 is a cylindrical or guideportion 46 which serves to align the tubing during the forgingoperation. A swage transition profile 48, having a radially outwardtaper extends from the lower end of guide section 46. This transitionprofile constitutes a mirror image of the transition section 36 of thefabricated inner tubing member 6. In the preferred embodiment of thisinvention, the taper of this transition section would be on the order of5°. Adjacent the transition section 48 is a swage tapered profile 50which corresponds to the tapered section 34 of the fabricated innertubing 6. Tapered profile 50 has a taper which is less than the taper oftransition profile 48, and in the preferred embodiment this swageprofile has a taper of approximately 1° to match the taper of thetransition section 34. At the lower end of the swage is a radiusedprofile 52. As with profile sections 48 and 50, the radiused profile 52is intended to match the cooperating section on the final inner tubingmember 6. Radiused section 32 of inner tubing 6 will be formed as theouter end of a standard upset J-55 tubing is forged by radiused profile52. It should be understood that although profile 52 is herein referredto as a radiused profile, it need not be generated by a constant radiusof curvature. The term "radiused profile" is merely intended to indicatethat the outward flaring of tapered section 36 generated by profile 52is significantly greater than that of adjacent sections 32 and 34 isinner tubing 6. It is believed that the term "radiused profile" isappropriate, however, since the actual profile would at least closelyapproximate a surface having a constant radius of curvature. Since theprincipal purpose of this radiused section is to provide radiallytraverse the separation between inner tubing 6 and outer tubing 4, aneffective radius of curvature on the order of magniture of the spacingbetween outer tubing 4 and inner tubing 6 should be effective to formthis profile. As can be seen in FIG. 3, the final shape of inner tubing6 can be fabricated by driving swage 42 into a standard tubular 38having enlarged or upset ends 40. Preferably, the portion of a standardtubular 38 adjacent the upset ends 40 would be heated prior to thisforging process. When the swage is driven into the end of the tubular,the tubular radially expands to form the flared end profiles desired forthe preferred embodiment of this invention. During the course of thisforging process, the ends of the standard tubular 38 would not only beradially flared but they would be stretched by the forging process. Asthe end is stretched the thickness of each tubular would be reduced. Theflared inner tubing 6 would have a radiused section 32 having athickness D₃, a tapered section 34 having a thickness D₄, and atransition section 36 having a thickness D₅. If the flaring andstretching of the material of the standard tubular is confined to theupset ends 40, the thicknesses, D₃, D₄ and D₅ can be greater than or atleast equal to the nominal thickness D₁ of a standard tubular. Even ifthe final thickness is slightly less than the nominal wall thickness ofthe tubing, the use of tubing initially having upset or enlarged endsshould promote greater structural integrity in the flared ends of thetubing. The stretching will, however, reduce the thicknesses D₃, D₄ andD₅ to a value less than the original thickness D₂ of the upset tubularends 40. A significant advantage to forming the inner tubing 6 from astandard tubular having upset ends can be seen in that even though thethickness of the standard upset ends is reduced, the thickness D₃ ofradiused section 32 can still be larger than the nominal thickness D₁ ofthe inner tubing member. This increased thickness should enhance thestructural integrity of the welds 30A and 30B along the radiusedsections 32 to the outer tubular member 4. The welds will extend over alarger surface area and the thickness of the inner tubing adjacent thewelds, including radiused section 32, tapered section 34, and transitionsection 36, will not be reduced below the nominal thickness of thetubing. This improved weld integrity would, in addition to the weldreliability improvement, be gained by reducing the number of welds ateach end.

After both ends of a single inner tubing member 6 have been flared bythe forging process depicted in FIG. 3, the final configuration of innertubing 6 will be that shown in FIG. 4. At this point, the reflectiveheat shield or low emissivity barrier can be applied to the outersurface of inner tubing 6. In the preferred embodiment, aluminum foilwould be wrapped around the inner tubing. The rigid insulation members14 may then be attached at appropriate positions along the exterior ofthe inner tubing by placing two half sections around the tubing withmetal bands securing the calcium silicate members together. Blanketinsulation 12 can then be attached over the remaining portion of innertubing 6.

The next step in the fabrication of the final insulating tubing conduit2 would be the insertion of the inner tubing-insulation assembly intoouter tubing 4. Upon insertion, the continuous circumferential surfaceformed at each free end of the flared inner tubing is portioned adjacentto the interior of the outer tubing around its complete innercircumference and is in position to be attached to the outer tubing. Theradiused end of inner tubing 6 can then be welded to outer tubing 4along one end of the concentric tubing assembly. This first weld 30Aextends completely around the junction between inner tubing radiussection 32 and the outer tubing 4. Multiple passes may be used to ensurethat this weld is structurally sound and completely seals the juncturebetween inner and outer tubing.

In the preferred embodiment of this invention, it is desired toprestress the tubing assembly by placing the inner tubing 6 in tensionand the outer tubing 4 in compression. This prestress is importantbecause of the loads which will be imparted to the conduit during hightemperature operation. The outer tubing, although in compression, wouldserve to maintain the inner tubing member 6 substantially in itsprestressed or preexpanded configuration. The length of the concentrictubing assembly should therefore be substantially the same in both thecooled and heated configuration. In addition, the stresses in theconcentric tubing assembly should be reduced during operation atelevated temperatures. After the first weld 30A has secured one end ofinner tubing to outer tubing, the desired prestress may be imparted bystretching the inner tubing 6 at the opposite end of the concentrictubing assembly. This stretching operation can be accomplished bymechanically pulling the inner tubing while holding the outer tubingfixed, or by heating the inner tubing relative to the outer tubing. Inthe preferred embodiment of this invention, the inner tubular member 6would not be initially prestressed beyond its yield point. After thedesired amount of prestress is imparted to the inner tubing, a secondweld 30B extending completely around the junction between inner tubingand outer tubing is made. Again, this weld may consist of multiplepasses to ensure the integrity of the weld.

Welds 30A an 30B have not only secured inner tubing member 6 to outertubing member 4, but have sealed the annular insulating cavity 13between the inner and outer tubing. In the preferred embodiment of thisinvention, it is desirable to increase the insulating capacity of thematerial in annular cavity 13 by withdrawing the gasses in annularcavity 13 to establish a vacuum. This vacuum may be established byinitially drilling a hole in, or otherwise piercing, the outer tubing 4to form an opening in the annular cavity 13.

A fixture 54, shown in FIG. 5, can be used for drilling a hole into theouter tubing 6 and for evacuating the gases from annular cavity 13. Thisfixture comprises a clamp 56 extending around the exterior of outertubing 6. A passage 68 extends through fixture 54 radially to the outersurface of tubing 6. A drill bushing, not shown, can be inserted intopassage 68 and an opening or hole 60 can be drilled into the outertubing 6 in alignment with radially extending passage 68. The samefixture can then be used to establish at least a partial vacuum inannular cavity 13 without losing alignment with the drilled hole 60. Thedrill bushing can be removed and a plug, such as a tapered pin,surrounded by an annular seal 62 can be inserted into passage 68, asshown in FIG. 5. A vacuum hose 58 can then be attached between fixture54 and a vacuum pump (not shown). Vacuum hose 58 communicates throughpassage 68 to the interior annular cavity 13. An O-ring seal 66, betweenvacuum fixture 54 and the exterior of concentric insulating conduit 2,prevents leakage during evacuation of annular chamber 13. The taperedpin 64 extending into passage 68 and the circumferential seal 62extending around tapered pin 64 prevent leakage through passage 68 pasttapered pin 64. After a suitable vacuum has been established withinannular cavity 13, tapered pin 64 may then be driven into drill hole 60to close that hole. The outer portion of pin 64 extending beyond thesurface of outer tubing 4 can then be removed, and, if necessary, a weldcould be employed to seal this pin.

After fabrication of the individual conduits, a plurality of conduitsmay be assembled to form an insulated tubing string by first insertingan internal coupling 20 in one end of each separate tubular members. Theinterior coupling is wedged into the flared end of inner tubular member6. Preferably, each interior coupling member 20 would be insertedfarther into one conduit than into the adjacent conduit. If the interiorcoupling member 20 is wedged into the tapered section 34 of one memberfarther than into the other, the interior coupling would remain affixedto a designated member upon disassembly. Field disassembly could then besimplified.

The preferred embodiment of this invention thus comprises a prestressedconcentric tubing member having thermal insulation along substantiallyits entire length. Convective, as well as radiant insulation, isprovided and the evacuation of the annular cavity between the twoconcentric tubing members removes residual moisture and reduces the heattransfer through the insulation. The preferred embodiment also employsonly two welds for each individual conduit. The integrity of the weldsemployed in this invention is increased by both reducing their numberand by employing flared inner tubing sections in which the thickness ofthe flared ends is not reduced below the nominal thickness of theintermediate sections of the inner tubing member. Furthermore,individual concentric insulating tubing members 2 have been fabricatedusing conventional tubing members.

Although the invention has been described in terms of the specifiedembodiment which is set forth in detail, it should be understood thatthis is by illustration only and that the invention is not necessarilylimited thereto, since alternative embodiments and operating techniqueswill become apparent to those skilled in the art in view of thedisclosure. Accordingly, modifications are contemplated which can bemade without departing from the spirit of the described invention.

What is claimed and desired to be secured by Letters Patent is:
 1. Aconcentric walled insulating tubular conduit for forming a tubularstring in a subterranean well to transport a heated fluid between thesurface of the well and a subterranean location while minimizing theheat loss from the heated fluid during transport, comprising: an outertubular member; an inner concentric tubular member having at least oneoutwardly flared end, said inner tubular member initially comprising atubular member with at least one enlarged upset end with a wallthickness greater than the nominal wall thickness of the tubular memberintermediate the ends, said outwardly flared end having a wall thicknessgreater than the nominal wall thickness of said inner tubular memberintermediate the ends thereof and less than the wall thickness of theinitially upset end; welded connections at both ends of the innertubular member to the outer tubular member with a single circumferentialweld joining a flared end of the inner tubular member directly to theouter tubular member at the flared end with greater wall thickness, saidinner tubular member being spaced from said outer tubular memberintermediate the welded ends thereof to define an annular insulatingcavity therebetween, said flared end on the inner tubular memberdefining one end of said annular insulating cavity, said weldedconnections forming a seal for said annular insulating cavity.